Wind turbines have received increased attention as a renewable energy source. Wind turbines use the wind to generate electricity. The wind turns multiple blades connected to a rotor. The spin of the blades caused by the wind spins a shaft of the rotor, which connects to a generator that generates electricity. Certain wind turbine systems include a doubly fed induction generator (DFIG) to convert wind energy into electrical power suitable for output to an electrical grid. DFIGs are typically connected to a converter that regulates the flow of electrical power between the DFIG and the grid. More particularly, the converter allows the wind turbine to output electrical power at the grid frequency regardless of the rotational speed of the wind turbine blades.
A typical DFIG system includes a wind driven DFIG having a rotor and a stator. The stator of the DFIG is coupled to the electrical grid through a stator bus. A power converter is used to couple the rotor of the DFIG to the electrical grid. The power converter can be a two-stage power converter including both a rotor side converter and a line side converter. The rotor side converter can receive alternating current (AC) power from the rotor via a rotor bus and can convert the AC power to a DC power. The line side converter can then convert the DC power to AC power having a suitable output frequency, such as the grid frequency. The AC power is provided to the electrical grid via a line bus. An auxiliary power feed can be coupled to the line bus to provide power for components used in the wind turbine system, such as fans, pumps, motors, and other components of the wind turbine system.
A typical DFIG system includes a two-winding transformer having a high voltage primary (e.g. greater than 12 KVAC) and a low voltage secondary (e.g. 575VAC, 690VAC, etc.) to couple the DFIG system to the electrical grid. The high voltage primary can be coupled to the high voltage electrical grid. The stator bus providing AC power from the stator of the DFIG and the line bus providing AC power from the power converter can be coupled to the low voltage secondary. In this system, the output power of the stator and the output power of the power converter are operated at the same voltage and combined into the single transformer secondary winding at the low voltage.
More recently, DFIG systems have included a three winding transformer to couple the DFIG system to the electrical grid. The three winding transformer can have a high voltage (e.g. greater than 12 KVAC) primary winding coupled to the electrical grid, a medium voltage (e.g. 6 KVAC) secondary winding coupled to the stator bus, and a low voltage (e.g. 575VAC, 690VAC, etc.) auxiliary winding coupled to the line bus. The three winding transformer arrangement can be preferred in increased output power systems (e.g. 3 MW systems) as it reduces the current in the stator bus and other components on the stator side of the DFIG, such as a stator synch switch.
Typically, the output voltage of the DFIG system on the primary winding of the transformer (e.g. a two winding transformer or a three winding transformer) can have a maximum continuous operating range of nominal voltage ±10%. Standard components of a wind turbine system which are powered by the auxiliary feed coupled to the line bus are typically designed to accommodate this range of nominal voltage ±10%. However, the operating range of new DFIG wind turbine systems has increased to accommodate a wider operating range on the primary of the transformer, such as nominal voltage ±15%.
A wider operating range on the primary winding of the transformer causes the voltage on the auxiliary power feed used to power components of the wind turbine system to have the possibility of being higher or lower than the ratings of the standard components powered by the auxiliary power feed. As a result, special components (e.g. components with higher ratings) may be required to accommodate the wider operating range. These special components can cost significantly more than standard components, and may require special qualification testing. In certain cases, special components that can accommodate a wider operating range may not be available at all, in which case major redesign of sections of the wind turbine system may be necessary. Consequently, providing a wider operating range on the primary of the transformer of the DFIG system (e.g. nominal voltage ±15%) can lead to significant drawbacks, including higher auxiliary system cost, longer development schedules, and other drawbacks.
Thus, a need exists for a system and method for improved voltage control in a DFIG wind turbine system. A system and method that can accommodate a wider operating range (e.g. nominal voltage ±15%) on the primary winding of the transformer while maintaining a standard operating range (e.g. nominal voltage ±10%) for the auxiliary power feed would be particularly useful.